The transport of ions or molecules across a biological membrane is a fundamental process in cellular life and is tightly regulated by ion channels, transporters and pores. Recently, researchers have adopted biological,1 solid-state,2 DNA origami3 and hybrid3a,b,4 nanopores in single-molecule analysis.5 Biological nanopores have advantages compared to their synthetic counterparts, mostly because they can be reproducibly fabricated and modified with an atomic level precision that cannot yet be matched by artificial nanopores. Biological nanopores, however, also have drawbacks. The mechanical stability of biological nanopores depends on individual cases. Alpha hemolysin from Staphylococcus aureus (αHL) and porin A from Mycobacterium smegmatis (MspA) nanopores remain open in lipid bilayers at high-applied potentials and can cope surprisingly well with extreme conditions of temperature,6 pH6b,7 and denaturant concentrations.6b,8 However, most of other porines and channels are much less robust. Arguably, however, the biggest disadvantage of biological nanopores is their fixed size. For example, the dimensions of αHL, MspA or aerolysin nanopores allowed the analysis of single stranded nucleic acids, aptamers or small peptides,9 but their small internal diameter (˜1 nm) precludes the direct investigations of other important biological systems such as folded enzymes or ribozymes.
Recently a significant number of studies sampled the translocation of folded proteins through artificial nanopores.10 However, the investigation of proteins with solid-state nanopores is difficult because proteins have a non-uniform charge distribution, they often adsorb to the nanopores surface and they translocate too quickly to be properly sampled.10c Further, because proteins have compact folded structure, the diameter of the nanopore should be similar to that of the protein.10b Recently, we have introduced Cytolysin A from Salmonella typhi (ClyA) as the first biological nanopore that allows the investigation of natively folded proteins.7a The ClyA structure is ideal for this task because proteins such as thrombin (37 kDa) or malate dehydrogenase (dimer, 35 kDa monomer) can be electrophoretically trapped between the wide cis entrance (5.5 nm, table 1) and the narrower trans exit (3.3 nm, table 1), and can therefore be sampled for several minutes. Ionic currents through ClyA are so sensitive to the vestibule environment that blockades of human and bovine thrombin can be easily distinguished.7a Our work was based on a ClyA construct where the two native cysteine residues of ClyA-WT (C87 and C285) were replaced by serine (ClyA-SS).7a However, ClyA-SS monomers showed low water solubility and low activity when compared to ClyA-WT monomers, and in planar lipid bilayers spontaneously opened and closed (gated) at applied potentials higher than +60 mV or lower than −90 mV.
Thus, there remains a need in the art for nanopore biosensors with high sensitivity for target analytes as well as high water solubility and stability at a range of potentials. Nanopore biosensors should have favorable properties of oligomerization, voltage dependent gating, and electrical noise in single-channel current recordings. The present disclosure relates to engineered nanopores in which specific substitutions to the native cysteine residues and other residues confer additional properties as compared with ClyA-WT and ClyA-SS.